From Sun Microsystems:
Java Runtime Environment (JRE) provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language. In addition, two key deployment technologies are part of the JRE: Java Plug-in, which enables applets to run in popular browsers; and Java Web Start, which deploys standalone applications over a network.
What's new in this version:
Version 7 Update 21 includes blacklisted jars and certificates, changes to Java control panel's security settings, changes to security dialogs, changes to RMI server JRE, and JDK for Linux on ARM.